The goal of the project is to determine the effects of familial environment, non-familial environment, and genetic factors on the etiology and course of drug abuse and dependence. There have been a number of family studies, one adoption study, and no rigorous twin studies of genetic factors in drug abuse/dependence. The identification of environmental factors involved in the etiology and maintenance of drug abuse/dependence has implications for treatment and prevention. The identification of genetic factors has potential for identifying high risk groups. The project will use the Vietman Era Veterans Twin Registry, comprising 6000 pairs, compiled by the National Academy of Sciences (NAS). Data will be collected by the Institute for Survey Research under subcontract to NAS. Data will be collected in 3 stages. The first stage will be a questionnaire mailed to all twins eliciting information on personal history and screening information on drug use and mood and antisocial personality disorders. In stages II, 4916 subjects, including all of those screened positive for drug use, will be included. Data collection entails structured telephone interviews for diagnosing drug abuse and dependence and mood and antisocial personality disorders. Stage III will include 2000 of the subjects using a mailed questionnaire to identify putative risk/protective factors. These data will be relevant for assessing if: (1) different drugs are influenced differently by familial, nonfamilial, and genetic factors; (2) different types of problematic use of the same drug are influenced differently by familial, nonfamilial, and genetic factors; (3) groups of drugs can be identified that are under the control of the same familial, nonfamilial, and genetic factors; (4) psychiatric morbidity mediates the genetic influence on drug abuse/dependence; (5) environmental risk/protective factors can be identified.